In comparison with an alternating current (AC) transmission system a HVDC transmission system requires only two conductors. At least one of those conductors must be isolated from earth. Hence, one of the conductors may be implemented as an overhead line or a high voltage cable. The other conductor may implement the use of earth or ground as the conducting medium. For bipolar transmission another conductor of the same kind is used under normal operating conditions. However, a ground conductor is required to transfer unbalance currents. The bipolar HVDC transmission system must also be capable of working as a monopole HVDC transmission system. During such operation the ground conductor transmits the whole DC current transmitted by the HVDC transmission system.
A HVDC transmission system comprises a first converter station connected to a first AC transmission network, a second converter station connected to a second AC transmission network and at least two conductors connecting the first and second converter station. Each converter station contains a grounding point connected to a ground electrode for making electric contact with earth. Thus the ground conductor comprises the ground electrode at each converter station and the ground between the two ground electrodes.
A bipolar HVDC transmission system conventially comprises a pair of overhead conductors carried by a plurality of towers from the first converter station to the second converter station. The conductors are suspended in isolators from arms of the towers. Each tower also caries shield conductors for protection of thunder lightning. Commonly the distance between two adjacent towers is around 400 m. Each tower is grounded to earth.
The ground electrode comprises a plurality of electrode elements for making electric contact with the ground. The ground is normally soil and/or sea water. Hence, the ground electrode may comprise a land electrode or a sea electrode. The electrodes operate as anodes, that is, delivering current to the conducting medium, or as cathodes, that is, receiving current from the medium.
A land electrode in this context is a ground electrode located in soil. The land electrode transfers the DC current from the HVDC transmission system to the soil or vice versa. The soil, in this context, is generally to be regarded as a conducting, however, inhomogeneous medium.
The land electrodes are, apart from the requirements as to current and resistance, also required to be electrically safe, to have high operational reliability and sufficiently long service life, and in addition, not to cause any harmful environmental effects, such as for instance drying up of the soil in the vicinity of the electrode.
In order to reach a sufficiently low grounding resistance, the land electrode usually comprises a large number of electrode element arranged in a tree structure. Hence, each electrode element is connected to the grounding point via a feeder conductor, a sub-feeder conductor and a plurality of further sub-feeder conductors. The aim is that the current flow shall spread evenly from the grounding point to each electrode element. In order to reach this aim each electrode element must have an equally long connection path the grounding point. Hence, every electrode element is connected as if they were connected to the grounding point in parallel.
In a known ground electrode arrangement the land electrode is connected to the grounding point of the converter station by a one or more feeder cables. Each sub-electrode is fed from a separate sub-electrode feeder cable. A sub-electrode comprises a backfill and an active electrode element embedded in the backfill. Most usually the backfill comprises a bed of coke. The electrode element is in electric contact with the sub-electrode feeder cable and comprises an active part of its surface which is in electric contact with the backfill. In cases where the sub-electrode comprises more than one such electrode element, these elements are coupled to each other by interconnection cables.
The backfill occupies a considerable volume around the electrode element and is in its turn embedded in the soil. The active part of the surface of the backfill is that part of its surface which is in electric contact with the soil.
From U.S. Pat. No. 6,245,989 (Iossel) a land electrode for high voltage direct current transmission system is previously known. The object of the land electrode is to improve the rate of dissolution of the feeding elements, and to improve the lifetime and reliability of the electrode. Hence the land electrode according to Iossel contains electrode elements separated from each other by electrically non-conducting element barriers.
A land electrode comprising sub-electrodes, sections and sub-sections may occupy a considerable piece of land. In a commonly known circular arrangement the diameter may be in the range of 500 to 1000 meter. Normally the land electrode is located distant from the converter station because the flow of DC current may harm the function of a nearby positioned transformer. A considerable piece of land is also occupied by the converter station and by the power lane containing the towers carrying the conductors between the converter stations.